The cholesterol-lowering effect of plant sterols has been known since the 1950's, and the literature of the field therefore includes several reports concerning the positive effects of plant sterols on the cholesterol level. New reports include for example the research by Heinemann et al., Eur. J. Clin. Pharmacol. 40 (Suppl. 1), 1991, 59-63, about the effect of sitostanol and sitosterol on the absorption of cholesterol in the intestine by using the intestinal perfusion technique. Sitostanol was found to reduce the cholesterol absorption by as much as 85% and the sitosterol absorption by 50%.
Nevertheless, the use of plant sterols has not been very extensive, due to for example their poor availability and therefore high price. One of the greatest problems has been the poor solubility of plant sterols, since they are not soluble in water and are also poorly soluble in fats. There have been efforts to solve the solubility problem by forming fat-soluble forms of plant sterols, for example fatty acid esters, and by using them in connection with fats.
GB 934,686 (Boehringer & Sohne) discloses a therapeutic preparation containing sitosterol, 1-4% by weight of gelatine or 0.1-2% by weight of methyl cellulose and a crystalline sugar as a filler. For preparation, sitosterol and sugar are ground and then combined in a blend of gelatine or methyl cellulose in water. According to said document the suspension obtained is no longer thixotrophic, which is very surprising, because gelatine and methyl cellulose are generally regarded as thickening agents. It is also mentioned that e.g. alginates or other cellulose derivatives do not have a corresponding effect. The invention described in GB 934,686 is, hence, based on the specific effect of either of two specific compounds, gelatine or methyl cellulose, in the sitosterol suspension.
An example of a method for preparing a fatty acid ester is the method disclosed in EP 195,311 (Yoshikawa Oil & Fat Co., Ltd.) wherein a plant sterol and a branched aliphatic primary or secondary alcohol having 14 to 32 carbon atoms is reacted with a fatty acid or fatty acid ester in the presence of lipase or cholesterol esterase in an aqueous solution or in a water-containing organic solvent. DE 2,035,069 (Procter & Gamble Co.) discloses the preparation of carboxylic acid esters of plant sterols with an acylation reaction by using perchloric acid as a catalyst. The obtained plant sterol ester is then added to a cooking or salad oil to produce an oil that reduces the cholesterol level.
WO 92/19640 (Raision Margariini Oy) discloses a .beta.-sitostanol fatty acid ester that is prepared by esterifying a .beta.-sitostanol mixture with a fatty acid ester or ester mixture in the presence of an interesterification catalyst. The obtained fatty acid ester is suggested to be used as a part of fats or oils in fat-containing products; a preferred embodiment is the addition of the ester to edible fats. The reference also discloses by way of comparison the effect of .beta.-sitostanol that has been emulsified into rape-seed oil on serum cholesterol levels. The result is not statistically significant and it is considered marginal as compared with the effect of the .beta.-sitostanol ester described in the reference.
EP 289,636 (Asahi Denka Kogyo Kabushiki Kaisha) relates to an emulsified or solubilized sterol composition wherein the sterols are emulsified or solubilized in an aqueous solution of polyhydroxy compounds containing sucrose and/or polyglycerol fatty acid esters or in liquid polyhydroxy compounds containing these fatty acid esters. The reference discloses that no solvent, such as fat or oil, is used for dissolving the sterols. However, the composition contains as an essential ingredient sucrose and/or polyglycerol fatty acid esters that are used in an amount of 0.5 to 10 parts by weight per part by weight of the sterol. The product is stated to be stable and useful for example in foodstuffs, cosmetics, drugs and agricultural chemicals. These final products are not described in greater detail.
U.S. Pat. No. 5,244,887 (Straub) discloses food additives containing plant stanols. The stands include sitostanol, clionastanol, 22,23-dihydrobrassicastanol, campestanol, and mixtures thereof. In order to prepare the food additive, a solution, suspension or emulsion of stanols is prepared by mixing the stanol with a solubilizing agent, an antioxidant and a dispersing agent. A preferred product is a composition containing about 74.8% vegetable oil, about 1.2% tocopherol and about 25% stanols. The additive is added to foodstuffs containing cholesterol, for example to meat, eggs and dairy products, and the purpose is to prevent the absorption of cholesterol from these foodstuffs.
The background art thus describes efforts to improve the solubility and therefore the range of use of plant sterols by forming fat-soluble derivatives from the sterols; the plant sterols and the aforementioned derivatives are dissolved or emulsified in a fat or a fat component and they are used as additives in fat-containing foodstuffs to prevent the absorption of cholesterol from these foodstuffs.